1. Field of the Invention
The present invention relates to a measuring tool, an encoder used for the measuring tool and a producing method of the encoder.
2. Description of Related Art
Various measuring tools for measuring distance, length and angle have been conventionally known. For instance, a caliper gauge is used as a measuring tool for measuring length. A caliper gauge has a main beam having a first measuring jaw, and a slider capable of slide movement along the longitudinal direction of the main beam and having a second measuring jaw to be in contact with a workpiece together with the first measuring jaw.
Maintaining measurement accuracy for a long time is of great importance for such a caliper gauge, and the main beam and the slider may preferably be made of material capable of minute processing and enduring repeated slide movement. Accordingly, the main beam and the slider of a caliper gauge are made of anti-frictional thermally treated metal having low linear expansivity such as stainless.
However, great amount of time and cost are required for machining metal down into a desired shape. In order to obtain good slidability between the main beam and the slider, the slide surface of the main beam and the slider has to be accurately machined by lapping etc. Such machining is also required for accurate measuring tool other than a caliper gauge. Since such measuring tool experiences many accurate processing during production process, the production line becomes complicated and production cost can be increased.
In order to maintain the measurement accuracy, linear expansion has to be kept within a predetermined range. In a general measuring tool, the error caused by the linear expansion is restrained by maintaining the temperature during measurement at twenty degrees Celsius, for instance. However, the measurement error by the linear expansion inevitably occurs in a severe measuring environment under hot and low temperature.
Further, when the measuring tool is grasped by hand, different linear expansion occurs on account of different temperature distribution by hand, which results in measurement error.
Further, a measuring tool having a slide mechanism such as a caliper gauge is likely to be worn after repeated use. Lubricant has to be used for reducing friction to maintain smooth slide movement, which is likely to cause adhesion of dust etc.
Digital measuring tool such as digital caliper gauge and digital micrometer has an encoder for detecting the movement of the movable member relative to the fixed member between the slide surface of the main beam and the slider.
As shown in FIG. 13, the encoder has a main scale 15 having an electrode pattern in which an electro-conductive portion 141 and an insulative portion 142 are alternately arranged along the longitudinal direction of a main beam 11 at a predetermined pitch, and a detector head 125 provided on a slider 12 to be electrostatically coupled with the main scale 15 to detect relative movement of the slider relative to the main beam.
The main scale 15 has an insulator 161 such as glass and polycarbonate and the electro-conductive portion 141 provided on the surface of the insulator 161 at a predetermined pitch. The insulator 161 is fixed (i.e. adhered) on the main beam 11 through a bonding layer 162.
The detected value of the detector head 125 is outputted to an electric circuit 126 to be arithmetically processed and displayed on a non-illustrated display as a measurement value.
However, since the main scale 15 is bonded on the main beam 11, the main scale 15 may be curved or the main scale 15 may be peeled off from the main beam 11, which can result in error in the detected value.
Further, linear expansion of the main scale 15 and uneven heat distribution by the heat of hand also occur as in a normal measuring tool.
The above problem is not restricted to a digital caliper gauge but also occurs in a measuring tool having an encoder for detecting relative movement of a movable component relative to a fixed component such as a digital micrometer and a digital dial gauge.